1. Field of the Invention
This invention relates to digital data processing systems, and particularly to such systems of the microprocessor size.
2. Description of the Prior Art
Within recent years the uses of data processing systems have expanded into a great variety of applications. With the number of uses increasing, the number and types of processing systems have also increased. Within the past decade, so-called minicomputers have begun to proliferate for use in tasks that do not require large scale processing systems. As with big systems, the minicomputers operate with changeable programs written in common programming languages such as Fortran and PL/1. The program flexibility of both the large and small computers is very useful in applications using continuously changing programs. The efficiency of such processing systems, however, is dependent greatly upon the efficiency of the Fortran program and of the Fortran compiler. For dedicated applications in which the programs remain constant, these systems are generally inefficient. It has been proven to be much more efficient to write dedicated control application programs in machine language on a smaller system designed for specialized applications.
A line of smaller data processing systems, termed microprocessors or microcomputers, has evolved to handle fixed-program applications. Since the program is fixed and not readily changed, it is stored in a read-only memory unit to protect against erasure or modification.
The advent of these microprocessors has been hastened by recent developments in integrated circuitry. A relatively large memory along with the data selection means for addressing the memory locations can now be formed on a single integrated circuit (IC) unit or chip. Arithmetic logic operators and data registers can also be formed on IC chips. Integrated circuitry has allowed microprocessors to be built smaller than previous minicomputers and as the length of the paths decreases, the processing speeds increase.
The architecture of prior microprocessors has tended to follow basic computer design, only on a smaller scale. Microprocessors have resembled small minicomputers. The reaction to the advent of integrated circuitry has been an attempt to combine as many elements as possible in a single IC chip. Some of the most recent microprocessors have been built completely on a single chip. The fabrication of such complicated IC chips, however, is very expensive.
While traditional computer architecture allows microprocessors to perform rapidly and accurately, it does not take full advantage of the capabilities of a small system. A simpler design may be able to accomplish the same results in a faster time. In addition, the cost of a microprocessor could be significantly decreased by making use of commercially available off-the-shelf IC packages instead of the complicated customized IC chips comprising the entire microprocessor.
Also, in accordance with traditional computer design, most prior microprocessors have a read/write memory for intermediate data storage in addition to the program storage read-only memory. These read/write memories outside the central processing unit reduce the processor speed because of the amount of time needed to address the memory and receive back the data.